Paper No. 145-4
Presentation Time: 2:40 PM
THE ORIGIN OF MODERN ATOLLS: CHALLENGING DARWIN’S DEEPLY INGRAINED THEORY
It is remarkable and somewhat disturbing that the Darwin model of the formation of atolls is widely accepted as correct, taught in introductory Earth science courses, included in most oceanography and Earth science textbooks, and is part of the definition of the term atoll on Wikipedia. In 1842, Darwin identified, in a publication, illustrated with multiple exquisite sketches and entitled "The Structure and Distribution of Coral Reefs", three types of reefs: fringing reefs, which are directly attached to volcanic islands; barrier reefs, which are separated from volcanic islands by lagoons; ring reefs, which enclose only a lagoon and are defined as atolls. Darwin linked those three types of reefs together in an evolutionary model that elegantly explained their interrelated origin through the slow and steady subsidence of low latitude volcanic edifices. It is important to remember that Darwin was unaware of Quaternary glaciations and their related high amplitudes/rates of sea level fluctuations. As an alternative, starting in the 1930s, several authors proposed the antecedent karst model; in this model, atolls formed as a direct interaction between subsidence and karst dissolution that occurred preferentially in the bank interiors rather than on their margins through exposure during glacial lowstands of sea level. Then, atolls developed during deglacial re-flooding of the glacial karstic morphologies by preferential stacked coral-reef growth along their margins. A comprehensive new model is proposed here, based on the antecedent karst model and well-established sea-level fluctuations during the last 5 million years, by demonstrating that most modern atolls, from the Maldives Archipelago and from the tropical Pacific and southwest Indian Oceans, are rooted on top of Late Pliocene carbonate flat-topped banks. The volcanic basement, therefore, has not influenced the late Quaternary development of these flat-topped banks into modern atolls. During the multiple glacial sea-level lowstands that intensified throughout the Quaternary, the tops of these banks were karstified; then, during each of the five mid-to-late-Brunhes deglaciations, coral reoccupied their raised margins and grew vertically, keeping up with their respective high amplitude sea-level rise and creating the modern atolls as we know them today.